1. Field of the Invention
The present invention relates to a diverting agent for blocking portions of a subterranean formation penetrated by the well bore whereby treating fluids are diverted to other portions of the formation.
2. Description of the Prior Art
Throughout the following disclosure, illustrative examples, and appended claims, the phrases "bridging agent" and "diverting agent" are employed many times. These two phrases are used interchangeably and each one defines and otherwise generally refers to an agent which functions to prevent the flow of liquid into a particular location, usually located in a subterranean formation, whereby the agent "bridges" on the particular location to seal the location and thereby cause the liquid to "divert" to a different location.
In the matrix treatment of subterranean oil and gas producing formations, wherein a treating solution is introduced into the formation, it is often advantageous to temporarily, but selectively plug portions of the formation with a solid material so that the treating solution is caused to flow to other portions of the formation. For example, in the matrix-acidizing of permeable formations, one of the most important factors effecting the success or failure of the acidizing treatment is the proper distribution of the treating acid over the entire producing interval. Due to variations in the permeability of a producing formation, the treating acid enters the most permeable portion which has the least resistance to flow thereby increasing the permeability of that portion and causing it to accept ever increasing quantities of acid. In order to prevent this uneven distribution of the acid, the acid must be diverted from the most permeable portions of the formation into the less permeable portions thereof.
In order to divert the treating fluid from the highly permeable portions of the formation into the less permeable portions of the formation, a volume of treating fluid is pumped into the formation followed by a diverting material (bridging agent in a carrier fluid) to seal off a portion of the formation where the first treating fluid penetrated and then following the diverting material with a second stage of treating liquid which is forced into another portion of the formation. The carrier liquid containing the bridging agent flows most readily into the portion of the formation having the largest pores, fissures, and vugs until that portion is bridged and sealed, thus diverting the remaining fluid to the next permeable portion of the formation. These steps may be repeated until the desired number of stages of treating fluid has been pumped.
A great variety of bridging agents have been used to plug formation openings and to divert treating fluids to other zones of the formation. For example, wax beads have been used as diverting agents. However, the wax beads have been found to have melting points, from about 138.degree. F. to about 192.degree. F., making them useless if the formation treating temperature exceeds their melting point. It has also been found that wax beads, upon melting will sometimes solidify the formation oil in which they dissolve unless they are present in the oil in concentration of less than one pound per gallon of oil.
Naphthalene particles have been used as diverting agents. Naphthalene is readily soluble in oil, melts at about 180.degree. F. and is suitable for only applications at lower formation temperatures.
Sodium chloride, which has a melting point of about 1,470.degree. F. has been used as a diverting agent. Sodium chloride, however, requires that the well be cleaned with water or dilute acid after the formation has been treated, if the salt is not dissolved by the produced formation water or water of the treating fluids. In addition, sodium chloride cannot be used with hydrofluoric acid to treat wells because of the insoluble precipitates which are formed.
Oil wells have been treated using grain blends as bridging agents. Typical grains used are corn, milo, millet, and peas. These grains, when used, as diverting agents, are difficult to remove in the formation and often require that the formation to be treated to be charged with nitrogen under great pressure before such diverting agents are used. The nitrogen blows the diverting agents from the formation openings when the well head pressure is released after the treatment.
U.S. Pat. No. 3,797,575, which is assigned to the assignee of the present invention and is hereby incorporated by reference, discloses diverting-forming additives comprised of relatively water insoluble solid material dissolved in a solvent such as methanol or isopropanol. When the additive is combined with an aqueous treating fluid, the solid material, dissolved in the additive, is precipitated in the aqueous treating fluid into a finally divided form. These precipitated particles act as a diverting agent.
U.S. Pat. No. 3,724,549, which is assigned to the assignee of the present invention and is hereby incorporated by reference, discloses a diverting material for diverting aqueous treating fluids into progressively less permeable subterranean formations comprising a carrier liquid and graded particles of cyclic or linear hydrocarbon resins having between about 20 and about 1,400 carbon atoms. The resin has a melting point of at least about 200.degree. F. and is acid and water insoluble but is oil soluble so that the resin can be removed by the produced oil after the treating operation has been completed.